1. The Field of the Invention
The present invention relates generally to ophthalmic devices, and more particularly, but not necessarily entirely, to digital ophthalmic devices.
2. Description of Related Art
Slit images were first used for examination of the human eye with the invention of the slit-lamp bio-microscope. These scopes were basically a microscope that allowed the eye tissue to be examined. With these devices, the tissue of interest was placed at the focal plane of the microscope. The first of these devices was invented by an ophthalmologist Alavar Gullstrand in 1911. It was found that if a narrow parallel sided mask was placed in front of the illumination source for the microscope, a slit could be formed and projected into the eye. The physicians found that the slit of light scattered into the tissue as it passed through the transparent tissues of the eye, just as the Tyndall effect is demonstrated with liquids or particles in space. This scattering is a change in spatial distribution of a beam of radiation when it interacts with a surface or a heterogeneous medium, in which interaction there is no change in wavelength or the radiation. This scattering or Tyndall images appear as a uniformly illuminated area that appears to be within the cornea of the eye. This scattering when positioned at various places on the eye would allow the physician to see a curved line of scattered light when looking into the eye. If the position and angle of the slit beam entering the eye was known, it was possible using trigonometry to calculate the relative thickness and distances of the tissues that were illuminated.
The next iteration of the bio-microscope had the mask arrangement built into the illumination system and also fixed the position of the masks such that their exact location was known with a common axis of rotation. Now when the eye was positioned into the focal plane of the bio-microscope, the slit images would also be positioned and the distances and angles easily be measured and known. It was then easier to calculate the distance relationships and thickness of the eye tissues. Because of this slit and lamp arrangement, and ability to closely measure eye tissue, the lamps became known as a slit lamp or slit lamp bio-microscope. Since its creation, the slit lamp bio-microscope has been used for very exhaustive exams of the eye, and over time several filters and attachments have been designed for use with this device, allowing the physician a tool that allows for a comprehensive exam of the eye. Along with more lenses and attachments the physicians have developed several ways to examine specific locations and areas of the eye depending upon the position of the lamp relative to the illumination source and also depending upon the type of illumination, either peripheral, backlighting or surface lighting. Each of these light arrangements yields a different set of images and lighting effects, which help isolate or remove the tissue reflectance, improving tissue scatter or transparency, used in determining an adverse or normal condition in the eye.
Light scatter within the ocular tissue has become a very integral part of this type of exam. The internal scatter of the tissue allows for a fairly uniformly illuminated cross section of the eye tissues. This phenomenon or the Tyndall effect was used in early devices called densitometers and employed in other scientific endeavors. This effect had found use in measuring the opacity or turbulence of liquids such as rivers or streams, angles or distances. The Tyndall effect is shown by understanding that light entering the stream or body of water would pass unimpeded if the water was pure and clean. If dirt, microscopic animal life or other substances were dispersed within the water sample, the light scatter caused by these objects could be seen by a image capture device, such as a camera. A comparison of a clouded image with scatter, against a clear image without scatter could be used to indicate a relationship of how much of a substance is dispersed in the water, where large concentrations are and what concentration is there. A simple sampling of the turbulent water with a count or analysis of the concentration of the material could be attached to the visual readings, creating a table or look up chart based upon scatter vs. concentration of material. Such work has been done in areas such as oceanography, pollution control, soil conservation and the like. This Tyndall effect has become an accepted method of estimating concentrations and distributions of materials within liquids or in air.
By simply applying the Tyndall effect to the study of the eye physicians have been able to create a device that can locate opacities within the human eye vitreous and in transparent tissues of the eye tissues such as with the crystalline lens and the cornea.
At about this same time as the slit lamp was being developed another major breakthrough in the examination of eye tissue was discovered by Theodore Scheimpflug in 1888. An Austrian, he developed, through the study of optics, what is now called the Scheimpflug rule. This rule describes an optical principle of constant focus through a lens, wherein the image and object plane are normal to each other, forming a 90 degree angle. The lens is placed midway between the two planes with the lens at the 45 degree angle. When this arrangement is established, the image and object remained focused on the image plane regardless of object position within its plane. With this new found arrangement, he went on to patent the effect, in 1904 for the use of ophthalmic examination and study. The use of this principle in the examination of the eye has yielded examination photographs of extreme image clarity and focus for tissues extending into the eye such as the crystalline lens.
Presently, some of the devices incorporating this technology are used for the examination of the lens and its opacities. The following work will help explain the theory and show images from this type of device Harold M. Merklinger: Scheimpflug's patent. Photo Techniques, November/December 1996. The first devices used for this type of examination are called scheimpflug cameras.
Scheimpflug cameras have been commercially in use for ophthalmology for the past 25 to 30 years. These devices are unique in that the eye is placed at the center optical axis of the device and at the center of rotation of a rotating head. A camera is placed to one side of the eye on a rotating head at a predetermined angle and is aimed and focused into the eye at a known angle. The rotating head projects a slit of light into the dilated eye of the patient. The camera is positioned such that the exam plane of focus (object plane) is at a right angle to the image plane focus. This preserves the focus of the system and ensures that all images are sharp and clear. During the exam, the slit light source is very intense and is powered by a xenon flash lamp to ensure an adequate amount of light. With each position of the rotating camera head and light source, an image is captured by a video camera. A set of images will be captured from 0 degrees rotation around the optical axis to 180. This amount of images will ensure that the optical portions of the eye are covered for examination. The images can be saved for viewing. The images are then shown and the position of the slit at various angles allows the physician to see into the eye of the patient at various angles, as if a cross section or slice of the eye were made at each angle. This device has been used extensively for the examination of the human eye, especially with regard to an exam of the crystalline lens in an effort to detect cataracts or opacities that form due to age or injury. This exam method allows the physician to take virtual slices of the patient eye for examination. The distances between objects in the eye, and size and thickness of the tissues in the eye are all able to be measured if the eye images are scaled to known and measurable units. Also the intense beams or slits used for the photography, illuminate imperfections in the eye causing scatter where opacities are present, and where air/liquid and tissue interfaces are located. This allows the physician to see, understand and locate any anomalies or imperfections of these tissues and their relationships in the eye.
With this interest in the internal tissues of the eye, other areas of interest in the optical mechanism of the eye such as optical aberrations such as focus or astigmatism requiring correction were also not to be ignored. In 1880, an inventor and physician Placido, found that if a target format of concentric light and dark rings, or mires, were reflected from the surface of the eye and observed, optical irregularities in the corneal surface could be observed. The invention, later called the Placido after its inventor, uses a reflection principle to observe these irregularities. The target or Placido is presented to the eye and the observer looks through a hole in the center ring or bulls-eye of the target. The viewer adjusts the distance to the eye until all mire rings are viewed in focus. Any irregularities in the corneal surface will be manifested by irregularities in the shape, spacing and concentricity of the reflected mires reflected from the patient eye. The appearance is as if a topographic map is made of the eye. If the cornea is spherical and smooth the mire rings will appear to be circular, evenly concentric and regularly spaced. If there are irregularities of the cornea, these will be observed by non-circular, irregular spaced and non concentric ring patterns. The further the mire rings appear from one another, a flattening of the cornea will be present, and if the there is steepening or more curvature change of the cornea, the mire rings will have closer spacing. Because the observed surface of the cornea, the anterior surface, is the surface that most controls the focus of the eye, any irregularities that are observed by direct observation or in images, can be used by the physician to determine and explain the optical aberrations observed by the patient. Devices based upon this technology have found widespread use for examination and diagnosis of eye anomalies. Its function for better and in depth eye measurement is limited however in that it is only a topographic device that examines the external surface of the cornea and that does not account for the corneal posterior surface contribution to the optical irregularities that are observed. Also there are several corneal anomalies which have been determined to occur in the posterior cornea which lead to aberration and vision failure that can not be readily observed with an anterior corneal measuring device. Because the cornea's posterior surface contribution to the overall vision prescription is about ½ of a diopter in correction, the physician will never fully arrive at a full understanding of the patient's corneal aberrations and how they affect the entire vision prescription for the patient. Additionally, the physician will be unable to observe corneal failures or other defects in early stages of development without a direct observation of the posterior surfaces of the cornea. Another drawback to these Placido-based mire ring devices is that due to the shape of the cornea, the measured shape of the eye can be tricked or mis-diagnosed by the physics of the design. Because we are observing reflected light, rays of light can appear to emanate from more than location on the mire ring or Placido. This problem that has been named the twist angle by developers in the field, is known and understood. Several manufacturers of these devices have undertaken creative solutions to help alleviate the problem, by using variously colored mire rings, square shapes, radiating line patterns etc. to overcome this error. Progress has been made in removing this latter problem, but there still remains the first problem, in that these devices as designed still can map only the anterior or front of the cornea. Another factor with using this type of device, is that the distance from the eye during image capture, has a significant effect upon the calculated distances. Such devices are calibrated by use of a reflective sphere of a known radius and image data that is captured is compared to the known data to determine the differences. With the differences known, the eye result is calculated based upon this difference. These devices are subject to error in that if the distance to the eye is slightly different than the calibration sphere was, the radius calculation for the difference in distance has an effect on the calculated sphere diameter, due to the differences of calibration and captured image. Thus with these Placido-based systems, it is critical that exact distances be repeated from subject eye to calibration sphere. Devices that employ this technology, have also undertaken methods to overcome this shortcoming such as triangulated dots on the cornea, extra rings etc.
As for measurement of the corneal thickness in order to understand and diagnose corneal anomalies, there has also been progress made. The first non-contact methods of corneal thickness measurement were performed with a slit lamp biomicroscope. In this process, a narrow slit of light is projected into the eye at a known angle forming a Tyndall image. A gracticule that is graduated in microns is placed in the eyepiece of the slit lamp. The width of the Tyndall image is measured with the graticule and by simple trigonometric calculations, a thickness can be determined. This type of examination is tedious and to get any true understanding of the overall corneal shape would require hours of examination and calculation time. Other non-contact methods are the Scheimpflug camera, which would also require a large amount of time for examination and calculation. Also, these devices, are designed to emit amounts of light into the eye, which allow for good visualization by the physician, but with long duration can be detrimental to the long-term health of the patient eye. Other than these methods to this time, the only other way to measure thickness was to use ultrasound measurements wherein changes of time delay from reflection of high frequency sound is waves are used to identify boundary layers such as the anterior surface of the cornea. The drawback to this methodology is that it is a contact method, and that as yet, there is not a way to measure truly and to calibrate devices to accurately measure in-vivo eye tissues. For non-contact thickness measurements, the field is quite narrow for choices open to the physician. However, within the last 12 years two significant devices for more thorough automated corneal examination have been developed and are on the market. These devices use a combination of technologies heretofore explained but gather and use this information in different ways.
One system, called the Orbscan,™ (Bausch & Lomb) is one such device. Originally designed as an optical densitometer, (Snook U.S. Pat. No. 5,512,966, Sarver et al, U.S. Pat. No. 6,120,150, Turner et al U.S. Pat. No. 5,864,383), it was found that this device could be used to map the locations of opacity and scatter within the eye. After experimentation it was found that the device could be used to locate boundaries of the scattered light and create maps of thickness if the distance between boundary layers could be calculated. This device uses a projection system, which includes a bulb, lens and a moving mask system. This mask is a parallel lined mask oriented vertically when projected onto the human eye. This mask, the object, is moved in a linear direction that is oriented in a plane that is normal to the image plane. The image plane for projection is located just inside the cornea of a human eye and is oriented parallel to the iris plane of the eye. This orientation between the object plane and image plane is established as a Scheimpflug relationship and follows the Scheimpflug rule. Due to this relationship, a linear motion of the mask yields an equal transverse motion of the slit light upon the eye, and while doing so remains in focus, on the pre-determined plane. Due to the Scheimpflug relationship, the image plane is always in focus. If an image capture device such as is used in the Orbscan, was to be focused to and was used to observe the image plane in the eye, all images that were captured could be focused easily and remain in focus during an exam.
When used for examination, the Orbscan device is placed at a predetermined focal distance away from the patient's eye determined by the optical properties of its lens system. Two controllable slit masks as previously described are placed on a plane each side of the optical system in the Orbscan, and are projected into the eye on an image plane as described, angling into the eye tissue at of 45 degrees off the center axis from each side. The video recording device, a camera is placed in between the mask projection systems on a central optical axis. The system is aligned by placing the optical axis of the system in alignment with the optical axis of the eye being examined while simultaneously being focusing the system at the image plane of the projected slit images. The images produced are Tyndall images which are illuminated cross sections of the subject eye, where in the scatter within the tissue is easily seen. The patient fixates his/her eye into the optical device system which brings the optical systems of eye and device in alignment. After being positioned thus, the exam commences. The illumination lamp output is focused by the condenser lens and focused through the mask, off of a mirror and into the eye tissue. the lamp illumination is commenced while each of the masks are positioned at pre-determined locations for starting. Following initial position, each mask is moved to another successive position fixed and moved again until each has completed its cycle of range positioning. Each position of the mask is controlled by a computer and hardware interface which synchronizes the image capture function of the system to the illuminated positions on the eye. There are 20 positioned Tyndall images taken for each lamp/mask combination, totaling 40 positions and 40 sets of Tyndall image data. The image data is then processed using proprietary algorithms to locate boundary edges based in the Tyndall images. Once boundaries are found, sizes, distances and physical relationships of eye tissue can be determined. The data is graphically rendered to form a mathematical representation of the eye and internal eye tissues. The information is then available to be displayed, printed and stored.
The drawbacks to this device involve these issues: 1) A dis-association of side data, from the true center data; 2) The amount of time required to take an exam, and mechanically position the mask for each successive position within the exam contributes to patient movement and eye motion increasing the error of the result; 3) The mechanical tolerances of mask position error cause variation and error in the final result calculation; 4) The change in angle of incidence as the light strikes the curved surfaces of the cornea is not calculated, and contributes to total system error; 5) The amount of time, effort and variation caused by human error during device assembly, alignment and calibration, also cause variations from one device to another; 6) Variations in the final resulting data due to changes of the opto-mechanical parts over time and use such as apertures, illumination intensity variations, positional location variation, and voltage variations and losses.
These six issues will be dealt with in order. The first disadvantage is the fact that the Orbscan device has two masks and projector systems each operating independently from one another. These masks and associated image capture systems each see the eye from a different respective location. The two systems are basically two different systems being linked together with a common base plate and capture system. This perspective of view creates two different viewpoints and makes it harder to help the curves appear to be on the same plane. Since they are also calculated separately and later joined, there are errors introduced into the surface calculations due to this average or combining that occurs. These surfaces essentially are two surfaces that are forced into alignment. The boundary areas between these surfaces can become distorted or even lost when these surfaces are joined. To help overcome some of this error, the Orbscan uses a Placido or ring reflection device for calculation of the outer boundary radius to which these two curved surfaces could be molded to.
This fix has a tendency to be prone to error as a Placido ring reflection device is very susceptible to distance location calculations from the eye. A small error in exact location of the Placido from the eye, changes the radius of eye curvature, thereby affecting the position and radius of the two disjointed surfaces. Another issue that is a drawback to the Orbscan device is the time taken to position a slit mask, settle it in position for an image capture, capture the image and move it again takes several seconds over the course of the entire exam. It is virtually impossible to keep the human eye still during this time. Regardless of the ability of a patient to remain still, the human eye moves in small random amounts called microsaccade motion. The eye performs this correction or motion an average of 60 motions per second. This motion allows the eye nerves to be stimulated while staring at objects and which allows the macula, or more sensitive part of the retina to observe the images better. During the time of the exam, the patient is fixated upon a target which requires staring or fixing of the eye. During this time the eye is moving with microsaccadic random motion without knowledge of the patient and without a detection method of the Orbscan. Because of this motion, it is virtually impossible for the eye to remain stable during the exam. This motion is a variable from one patient to another, adding to the variance from patient to patient and from one eye to another. Another factor that contributes to error is that the eye actually changes diameter during the exam due to the heart rate and ocular pressure of the patient. These variances force the calculations to be more averages rather than exact measurements of the eye and eye tissues. The physicians using these devices require accurate information with which to prepare a surgical treatment or diagnose a malady or even determine if the eye is normal. Minute changes, errors or variances in this information can cause optical parameters to change thereby affecting the outcome. The Orbscan device has been shown to have variances from machine to machine. The physicians are demanding that the variance be removed as far as possible from the measurement results.
The next issue is the fact that the Orbscan device uses mechanical stepper motors and slides to create a linear motion and position system for the masks. These individual components are subject to commercial manufacturing tolerances and these tolerances vary from one component to another. When all these components are brought together, they create a background amount of variability that is important to overcome. The Orbscan device uses a calibration process to unify the components for each individual machine. This data variation is supported as there is consistency within a machine, but variance from machine to machine. The population of machines produced can be affected by a change in tolerances and such tolerances will tend to show up as variation in the final result. Consequently, we see these variations in measurement and in statistical results. Other issues that can affect performance of this device is the relative change in incidence angle and its affect on the calculations for thickness and location of ocular tissues in the eye. The human eye is not a flat surface and as such the light entering the eye from a fixed location and angle is subject to the variations of the corneal shape. These variations will be evidenced by variations in the corneal thickness across the eye even if these variations in the actual tissue don't exist. This aspect of the Orbscan measurements are not taken into consideration, as the present system does not have the capability to do so. The two remaining drawbacks are that mechanical performance of devices change over time. These variances may be due to shipment vibration, wear, age of lubricants, dust, filament aging due to use, etc. Each of these issues are a variable which affects the long term performance of the device and its accuracy. Additionally, much of the adjustment required for the device is external manual adjustments. This variability from assembler to assembler will affect the baseline performance for the device, both individually and statistically across the population.
The Pentacam™ (Oculus) device is basically a diode illuminated Scheimpflug camera. This device is a small device based upon the previous technology used in Scheimpflug photography. The theory behind this device is the same as the other previous Scheimpflug devices. The difference is the application of a diode illumination system instead of the flash lamp. The diode illumination system has a real advantage over traditional incandescent light sources. The wavelength in this device is ranged more toward the shorter wavelength or blue end of the visual spectrum, which has a tendency to create more scatter within the tissues of the eye. The other advantage to this system is that the images produced are deep, meaning they range from the cornea to the back of the lens, and they are highly focused. This device exam is done the same way as the older Scheimpflug camera, in that the patient is positioned in front of the optical head with the eye on the optical axis. The exam commences and the camera is rotated around the eye at 5-10 degree intervals for 180 degrees. The data is captured and the surfaces are calculated. This device has found great acceptance however there are drawbacks as we shall discuss. The exam is shorter in duration than the Orbscan device but is also long enough that the eye motion is a factor.
The drawbacks to this technology are 1) there is a problem of determining the true center axis to rotate around. If the eye moves at all, it is lost. 2) the time for the exam takes too long allowing eye motion to be a factor in the results. As the eye is focused at the center, this device requires an axis center to rotate around. The high point of the cornea, or apex of the eye is not always the accurate place as it is sometimes off the center and not on the optical axis. In most human subjects the iris plane is not normal to the optical axis of the eye. This is referred to as angle kappa. This aspect of the eye orientation to bring the macula into position for good vision, causes a point that is not the apex of the cornea with respect to the iris plane to be the high point. Because of this, an artificial rotational axis has to be located and used. This hopefully is aligned with the optical axis of the eye. This axis can be created by reflex reflections on various surfaces to align by, but if the eye moves a small amount, this point is moved and consequently all maps that require the center to be matched to cannot be accurately mapped and placed upon the center. This causes the software to determine an average point to set all exams to. With this average location, a compromise in the surfaces will be obtained, and results will not be accurate. This has actually been witnessed in the use of this device. The photographs and images are pristine; the data has less than expected accuracy.
Several devices have been developed to increase the accuracy of the above devices such as using a mask controlled by an LCD array. This is a novel approach but is flawed in that the devices proposed, provide diffraction at each aperture. The apertures being small have a tendency to diffract the light making the image more irregular and non uniform. The other drawbacks are that the device polarizes light as it exits. Polarized light may provide one direction of light scatter, but may prove to have high loss in other directions or on other surfaces. This has yet to be proved. Another issue is the amount of efficiency of the light traveling through the apertures yields a loss on the order of 60 percent. This makes the machine more un-gamely and large due to increased lamp size and efficiency.
This discussion of these devices shows that the data obtained from present devices needs accuracy to place the surfaces in an orientation and location that is accurately positioned with regard to the optical axis of the eye. This error in data and mapping is not possible due to the time required for exams, and due to the fact that the data maps do not have a method of tying the data together that is accurate. Also we see that the data gathered is related to the manufacturing tolerances and positional error due to mechanical controls.
What is needed is a device that eliminates as much as possible any mechanical device for positioning and also allows quick fractional second acquisition of the data. There also has to be a method of tying the data surfaces together in order to provide a mesh of data that does not rely on the positional location obtained by tracking of the eye. Also there needs to be a feature that allows the device to accommodate differing colors of irises and allow an overall exam of the eye for sizing and metrics. Other features not present in the previously available designs such as internet upgrades, different custom patterns would also be desirable.
The features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.